One of the applications of gear pumps is in the mobile refuse removal industry. In such applications, the gear pump is typically mounted off the crankshaft in front of the engine and is operational whenever the engine is running. By mounting the gear pump in such a manner, the use of transmission power take-offs is eliminated, thus saving the cost of the power take-off and the plumbing associated therewith. In such mobile refuse removal applications, hydraulic pressure is typically required for short, intermittent periods of time, but the operator does not have the ability to disengage the gear pump when not in use. The foregoing problem is minimized by the use of a dry valve assembly attached to the inlet of the gear pump. When hydraulic pressure is required, the dry valve is opened permitting the flow of hydraulic fluid through the valve into the gear pump and out into the hydraulic work circuit. Conversely, when hydraulic pressure is not required, the dry valve is closed preventing the flow of hydraulic fluid therethrough to the gear pump. Since the gear pump is running at all times, lubrication of its internal components (gears, bearings, etc.) is required. When the dry valve is in the open position, such lubrication is effected by the flow of hydraulic fluid through the gear pump to the hydraulic work circuit. When, however, the dry valve is in the closed position preventing the flow of hydraulic fluid to the pump, lubrication of the foregoing internal components must be effected by other means. In prior art systems, such lubrication is effected by means of a small aperture within a shut-off plate in the dry valve. The foregoing aperture permits a limited flow of hydraulic fluid therethrough to the gear pump even when the dry valve is in the closed position. Since a given amount of hydraulic fluid is being drawn into the gear pump, a similar amount of hydraulic fluid flow must be discharged therefrom. In the prior art systems, the hydraulic fluid flow from the pump has a discharge pressure sufficient to cause cavitation resulting in the degradation or the removal of metal from internal gear pump components and the creation of abnormally high noise levels. The amount of cavitation depends upon the discharge pressures attained. To minimize cavitation the prior art systems utilize a pressure compensated bypass valve mounted externally to the gear pump-dry valve assembly and fluidically interconnecting the outlet port of the gear pump to the hydraulic work circuit. Any flow of hydraulic fluid within the gear pump for lubrication purposes while in the "dry" mode of operation passes directly through the bypass valve to the oil system reservoir thus eliminating back pressure build-up which, in turn, minimizes cavitation and the noise associated therewith. One of the primary disadvantages of using a pressure compensated type bypass valve is that it "bleeds" off a given amount of hydraulic fluid during the entire work cycle reducing the output horsepower of the gear pump. In addition, since the pressure compensated bypass valve is not an integral part of the gear pump, installation of the complete gear pump-dry valve system requires additional time for the mounting of the bypass valve.
Because of the foregoing disadvantages associated with the prior art systems, it has become desirable to develop a gear pump system wherein the structure of the dry valve is relatively simple and compact and wherein the bypass valve is an integral part of the gear pump and is not of the pressure compensated type.